This application relates generally to semiconductor device fabrication, and specifically to preparation of TEM samples for testing internal structure and chemistry of fabricated devices.
Transmission electron microscopes (TEMs) were developed because of the resolution limitations of light microscopes, which are imposed by the wavelength of visible light. TEMs are among the most useful and versatile tools for characterization of materials. In contrast to scanning electron microscopes, which only image the surface of a material, TEM allows analysis of the internal structure of a sample.
As critical dimensions of integrated circuits have become smaller and more complex, TEM analysis has been relied on as an essential technique for high spatial resolution imaging at the atomic level. TEM samples must be transparent to the electron beam in order to image the internal structure of the sample. In terms of a TEM, xe2x80x9cthinxe2x80x9d means the specimen must transmit sufficient electrons such that enough intensity falls on the screen or photographic plate to create an interpretable image in a reasonable time. This is a function of the electron energy and the size and weight of the atoms comprising the specimen. Though higher beam energy allows thicker samples to be imaged, the danger of damage to the sample from the electron beam increases. High sample yield and fast turn-around time are also important economically.
Samples are typically thinned by cutting out or grinding down a tiny piece of the specimen which is further thinned by an ion milling process or use of a focused ion beam. The mechanical thinning (i.e., the cutting or grinding) is required because ion beams typically remove strips of material with thicknesses in the tens or hundreds of nanometers, and large scale thinning using such precise devices is time consuming and therefore expensive. Samples prepared this way can reach thicknesses of only a few hundred angstroms.
Focused ion beams (FIBs) used in ion milling (which bombards a material with ions to remove parts of the targeted material) accelerate ions using electric fields. A variety of ion species may be used, including Ga, Si, Au, Co, and Pr. Focused ion beam methods can be used for implantation, sputtering, deposition, micro-machining, and ion beam lithography, depending on the setup and the energies used. High resolution of FIBs allows identification of precise areas to be sampled, which is very important in TEM since only a tiny relative area may be viewed due to the magnification levels used. Using FIBs to thin samples also gives operators better control than mechanical polishing techniques, resulting in higher yield. Most FIB techniques provide reliable and repeatable results for routine analysis.
An example of using FIB to prepare samples is found in a paper by Morris et al., xe2x80x9cA Technique For Preparing TEM Cross-Sections to a Specific Area Using the FIB,xe2x80x9d Proceedings of ISTFA 1991, pp. 417-427, which is hereby incorporated by reference. First the sample is mechanically thinned to make the FIB use practical. The FIB removes the remainder of unwanted material, leaving an electron transparent wall. The process uses optical means to choose an area for sampling, and mechanical lapping reduces the area to a thickness of about 30 micrometers. Either one-sided or two-sided ion milling is used to further thin the material down to thicknesses of less than several hundred nanometers.
Use of FIB methods in sample preparation has reduced the time required to prepare samples for TEM analysis down to only a few hours. However, for today""s stringent device requirements, one sample alone is often not enough to sufficiently characterize and qualify a specific process. When multiple samples are taken, a few hours sample preparation time can turn into days or even weeks.
A different method of TEM sample preparation has been reported using electron beam lithography and reactive ion etching (RIE) to etch out a sufficiently thin sample. An example process can be found in a paper by Wetzel, et al., xe2x80x9cOn the Preparation of Cross-Sectional TEM Samples using Lithographic Processing and Reactive Ion-Etching,xe2x80x9d Ultramicroscopy v.29, pp. 110-114 (1989), which is hereby incorporated by reference. A photoreactive compound is spun on the substrate and cured, followed by exposure with an electron beam lithographic device. After development, the process leaves a stencil of the e-beam exposure. This mask has a selectivity difference with the underlying material to be sampled, so that during RIE the mask is nearly consumed but the sample is etched to define a thin electron transparent wall. The selectivity of the mask material used determines the height of the sample wall. This process, though reported years ago, has not been adopted for wide-scale use because of high cost in tools and masks required.
There is therefore a need in the art for a process of preparing TEM samples that requires less preparation time to make TEM sampling a viable part of semiconductor analysis and manufacturing.
Mass Production Cross-section TEM Samples by Focused Ion Beam Deposition and Anisotropic Etching
The present application discloses a method of producing cross-section TEM samples using a focused ion beam to deposit a mask and an anisotropic etch process to etch around the mask. The preferred embodiment uses focused ion beam deposition and reactive ion etching, as follows. The region of interest is imaged using a scanning electron microscope. The FIB is used at a low power to deposit a thin strip of platinum (though other mask materials can be used) which acts as a mask when surrounding material is removed by etching. During etching, material on both sides of the platinum strip are removed in a single etch process, leaving a thin wall that is transparent to electron transmission sufficient for imaging through a TEM. After RIE, the sample is ready for TEM.
Though the preferred embodiment uses reactive ion etching, any low discharge etching process that has the required anisotropy can be used.
Advantages of the disclosed methods and structures, in various embodiments, can include one or more of the following:
saves preparation time of multiple TEM samples;
samples used for measuring critical dimension can be obtained from many sites within a die as well as from different dies of a wafer.